1

U.S. Geological 

Survey

Water-Resources 

Investigations 

Report 97–4006

1997

Abstract

Instream nutrient loads of the Altamaha, Suwannee, St. Johns, Satilla, 

Ogeechee, Withlacoochee, and Ochlockonee River basins were computed 

and compared with nutrient inputs for 

each basin for the period 1986-90. 

Nutrient constituents that were considered included nitrate, ammonia, 

organic nitrogen, and total phosphorus. Sources of nutrients considered 

for this analysis included atmospheric 

deposition, fertilizer, animal waste, 

wastewater-treatment plant discharge, 

and septic discharge.

 The mean nitrogen input ranged 

from 2,400 kilograms per year per 

square kilometer (kg/yr)/km2

 in the 

Withlacoochee River Basin to 5,470 

(kg/yr)/km2

 in the Altamaha River 

Basin. The Satilla and Ochlockonee 

River Basins also had large amounts 

of nitrogen input per unit area, totaling 

5,430 and 4,920 (kg/yr)/km2

, respectively.

Fertilizer or animal waste, as 

sources of nitrogen, predominated in 

all basins. Atmospheric deposition 

contributed less than one-fourth of the 

mean total nitrogen input to all basins 

and was consistently the third largest 

input in all but the Ogeechee River 

Basin, where it was the second largest.

The mean total phosphorus input 

ranged from 331 (kg/yr)/km2

 in the 

Withlacoochee River Basin to 1,380 

(kg/yr)/km2

 in both the Altamaha and 

Satilla River Basins. The Ochlockonee 

River Basin had a phosphorus input of 

1,140 (kg/yr)/km2

.

Per unit area, the Suwannee River 

discharged the highest instream mean 

total nitrogen and phosphorous loads 

and also discharged higher instream 

nitrate loads per unit area than the 

other six rivers. Phosphorus loads in 

stream discharge were highest in the 

Suwannee and Ochlockonee Rivers. 

The ratio of nutrient outputs to 

inputs for the seven studied rivers 

ranged from 4.2 to 14.9 percent, 

with the St. Johns (14.9 percent) and 

Suwannee (12.1 percent) Rivers 

having significantly higher percentages than those from the other basins. 

The output/input percentages for 

mean total phosphorous ranged from 

1.0 to 7.0 percent, with the St. Johns 

(6.2 percent) and Suwannee (7.0 

percent) Rivers exporting the highest 

percentage of phosphorous. 

Although instream nutrient loads 

constitute only one of the various pathways nutrients may take in leaving a 

river basin, only a relatively small part 

of nutrient input to the basin leaves the 

basin in stream discharge for the major 

coastal rivers examined in this study. 

The actual amount of nutrient transported in a river basin depends on the 

ways in which nutrients are physically 

handled, geographically distributed, 

and chemically assimilated within a 

river basin.
2

Introduction

An activity undertaken in the GeorgiaFlorida Coastal Plain (GAFL) study unit (fig. 1), 

as a part of the U.S. Geological Survey (USGS) 

National Water-Quality Assessment (NAWQA) 

Program, was to examine available surfacewater-quality data for nutrients (nitrogen and 

phosphorus compounds) in the study unit. 

These compounds were selected because they 

can cause eutrophication of surface waters, and 

because of the possible health risk associated 

with the nitrogen compound, nitrate, in drinking water. 

As interest in quantifying nitrogen and 

phosphorus levels in surface waters has 

increased, interest in describing and quantifying the sources of these nutrients has also 

increased. Point sources, such as wastewatertreatment plants and industrial outflows, are 

obvious sources of nutrients. Important 

nonpoint sources of nutrients include animal 

wastes, fertilizer from agricultural and urban 

settings, discharges from septic-tank systems, 

and atmospheric deposition. Comparison of 

instream nutrient loads among river basins 

provides information that can be used by 

water-quality managers to implement effective 

nutrient reduction plans.

This report presents information on 

instream nitrogen and phosphorus loads during 

1986-90 in seven rivers of the GAFL NAWQA 

study unit. This information is compared with 

input loadings from point and nonpoint sources 

of these nutrients within each of the studied 

river basins.

The seven large river basins in the 

GAFL study unit that were selected for study 

are referred to by river name for convenience. 

However, there are two Withlacoochee Rivers 

in the GAFL study unit, one in central Florida 

and one in southern Georgia. The basin of the 

Withlacoochee River discussed in this report is 

that of the river in central Florida (fig. 1). 

Physical, hydrologic, and land-use characteristics are described for each basin and listed in 

table 1; basin boundaries are shown in figure 1. 

STUDY UNIT BOUNDARY

RIVER BASIN BOUNDARY

SURFACE-WATER

GAGING STATION

EXPLANATION

FLORIDA

MEXICO

OF

GULF

ATLANTIC

OCEAN

ALABAMA

GEORGIA

CAROLINA

SOUTH

28°

29°

30°

31°

32°

33°

34°

85° 84° 83° 82° 81°

100 MILES

0

0

100 KILOMETERS

Location

of

Study

Area

FLORIDA

GEORGIA

SOUTH

CAROLINA

ALABAMA

Savannah

Tallahassee

Jacksonville

Brunswick

Atlanta

Orlando

St. Petersburg

Tampa

Tifton

Macon

Athens

Valdosta

SUWANNEE

RIVER BASIN OCHLOCKNEE

RIVER BASIN

SATILLA

RIVER BASIN

OGEECHEE

RIVER BASIN

WITHLACOOCHEE RIVER BASIN

ALTAMAHA

RIVER BASIN

ST. JOHNS

RIVER

BASIN

Figure 1. Georgia-Florida Coastal Plain study unit and the seven river 

basins that were studied.

Table 1. Basin characteristics

[km2

, square kilometers; m3/s, cubic meters per second]

1

Data for all land use categories are not presented. Rangeland comprised 19.1 percent of the St. Johns River Basin, and 6.5 percent of the Withlacoochee River Basin. 

Data source GIRAS (Mitchell and others, 1977). 2

Discharge for this site obtained from the gage near Doctortown, Ga.

Station name

Drainage area

at gage 

(km2)

Period of 

record for 

discharge data

Average 

discharge 

(m3

/s)

Land use (percent of basin area)1

Forest Wetland

AgricultureResidential

Commercial, industrial, 

other urban

Altamaha River at Everett City, Ga.2 36,300 1932-90 382 64.2 4.8 26.4 2.4 0.9

Suwannee River at Branford, Fla. 20,400 1932-90 196 50.4 15.9 31.2 0.8 0.9

St. Johns River near De Land, Fla. 8,690 1934-90 86.2 16.5 24.9 26.9 3.9 1.7

Satilla River at Atkinson, Ga. 7,220 1931-90 63.4 55.1 12.7 30.9 0.7 0.3

Ogeechee River near Eden, Ga. 6,890 1938-90 64.6 48.6 12.0 37.8 0.5 0.3

Withlacoochee River near Holder, Fla. 4,730 1932-90 30.0 16.1 27.3 40.0 2.9 1.0

Ochlockonee River near Havana, Fla. 2,950 1927-90 29.4 44.7 2.3 48.9 1.3 0.8
3

A comparison of the hydrologic conditions 

was made using the values of mean runoff and 

mean water yield (ratio of runoff to precipitation expressed as a percentage), which were 

computed for 1986-90 for each of the seven 

drainage basins (table 2).

Approach

USGS water-quality monitoring stations 

with at least 20 years of discharge data and 10 

years of nutrient data were used to select the 

seven rivers that were examined in this report. 

The station located farthest downstream in each 

river basin was chosen to establish the downstream limit of the study basin for each river. 

The boundary of the basin contributing runoff 

to the farthest downstream station in each study 

basin was digitized from 1:24,000 or 1:100,000 

topographic maps. The digitized areas of six of 

the basins corresponded well to values previously published in USGS annual data reports 

(U.S. Geological Survey, 1994), with differences ranging from 0.01 to 2.5 percent. However, the digitized area of the St. Johns River 

Basin was 9 percent larger than previously 

reported (U.S. Geological Survey, 1994). This 

difference was apparently caused by construction of drainage canals for residential and agricultural development during recent years, 

resulting in artificial expansion of the southeastern part of the St. Johns River Basin. To facilitate comparison of data among the basins 

(because drainage areas differ approximately 

tenfold) nutrient input and output values were 

divided by the area of their respective basin to 

obtain a nutrient yield.

Methods for aggregating and assessing 

nutrient inputs and outputs for this report are 

discussed below. Within each river basin, nutrient input loads were computed for atmospheric 

deposition, fertilizers, animal waste, septic-tank 

effluent, and wastewater-treatment plant discharges. Instream nutrient loads were computed 

for each of the seven selected rivers.

Data on weekly wet deposition of nitrate 

and ammonium for 1986-90 were obtained for 

nine sites (fig. 2) in the National Atmospheric 

Deposition Program (NADP) in Florida, Georgia, Alabama, South Carolina, and North Carolina (National Atmospheric Deposition Program (NRSP-3)/ National Trends Network, 

1993), and summarized by year. Dry (particulate, aerosol, and gaseous) deposition is also an 

important additional atmospheric source of 

nitrogen (Baker, 1991). Dry deposition of nitrogen in the basins was estimated by multiplying 

the estimated wet deposition by 0.96, which is 

the mean estimate of the ratio of dry deposition 

to wet deposition for Florida (Baker, 1991). 

Atmospheric deposition of nitrogen was estimated as the sum of dry and wet deposition. 

Concentrations of organic nitrogen and phosphorus are not collected as part of the NADP 

and, therefore, those nutrient components were 

not included in the computations.

The Thiessen polygon method (Gilman, 

1964) was used to estimate annual atmospheric 

deposition loading for nitrogen to the study 

basins. Thiessen polygons were intersected with 

the drainage basins to assign atmospheric deposition to each basin. Where more than one 

Thiessen polygon intersected a drainage basin, 

atmospheric deposition was assigned proportionate to the area it covered in that basin, and a 

basin deposition value was derived from an area 

weighted average.

Data for manure and fertilizer inputs are 

reported for 1986-90, but not all data were 

available for all years. The manure and fertilizer 

data were accumulated annually for the period 

of July 1–June 30, rather than by calendar year 

or water year, whereas other data were available 

on a monthly or weekly basis. Inputs of nutrients from crop fertilizers were estimated from 

summaries of annual fertilizer sales (as nitrogen 

and phosphorus) by county for Georgia and 

Florida (Janice T. Berry, Tennessee Valley 

Table 2. Rainfall, runoff, and water yield for the seven study basins (1986-90) 

River basin

Mean rainfall

(centimeters

per year)

Range of

yearly

rainfall

(centimeters)

Mean runoff

(centimeters 

per year)

Range of

yearly

runoff

(centimeters)

Mean runoff/

mean rainfall

(percent)

Altamaha 114 95-130 27 16-40 24

Suwannee 120 97-132 30 15-45 25

St. Johns 109 83-146 25 18-36 23

Satilla 114 98-133 26 5-47 23

Ogeechee 113 86-129 24 12-36 21

Withlacoochee 124 100-142 14 7-18 11

Ochlockonee 127 102-156 34 8-66 26

Figure 2. Atmospheric deposition sites and Thiessen polygons used to estimate 

annual deposition and atmospheric nutrient loads to each basin.

FL14

02329000

THIESSEN POLYGON

STUDY AREA BOUNDARY.

RIVER BASIN BOUNDARY.

SURFACE-WATER GAGING

STATION—And site

identification number.

ATMOSPHERIC DEPOSITION

SITE—And site

identification number

EXPLANATION

28°

27°

29°

30°

31°

32°

33°

34°

35°

88° 87° 86° 85° 84° 83° 82° 81° 80°

0

0

100 MILES

100 KILOMETERS

FL41

FL14

GA23

FL03

FL99

GA41

AL10

AL99

NC25

SC06

GEORGIA

NORTH CAROLINA TENNESSEE

ALABAMA

FLORIDA

SOUTH

CAROLINA

Ochlockonee

River Basin

Withlacoochee

River Basin

St. Johns

River Basin

GA50

GA20

Ogeechee

River Basin

Altamaha

River

Basin

Satilla River

Basin

Suwannee

River Basin

02236000

02202500

02313000

02320500

02329000

02226160

02228000
4

for each basin were multiplied by the average 

nutrient concentrations in wastewater, classified 

by waste type, to estimate loadings of nitrogen 

and phosphorus (National Oceanic and Atmospheric Administration, 1993). Some of the 

wastewater discharge data were incomplete or 

missing, resulting in underestimates of inputs 

from this source.

Instream nutrient loads were estimated on 

an annual basis using daily discharge and nutrient data obtained from the USGS National Water 

Information System for each of the seven 

selected stations. Nutrient concentrations were 

modified by aggregating data for related parameters (such as total and dissolved concentrations) 

which were not statistically different between 

samples when replicate samples were analyzed 

(Mueller and others, 1995). In this report, the 

term ammonia refers to the combination of total 

and dissolved ammonia as nitrogen. The term 

nitrate refers to the combination of total nitriteplus-nitrate as nitrogen, total nitrate as nitrogen, 

dissolved nitrite-plus-nitrate as nitrogen, and dissolved nitrate as nitrogen. Instream loads of 

ammonium, nitrate, total Kjeldahl nitrogen 

(TKN) and total phosphorus were estimated by 

computer software, implementing the minimum 

variance unbiased estimator model (Cohn and 

others, 1989; Gilroy and others., 1990). This 

model estimates monthly basin nutrient loads as 

a function of sampled concentrations, time, discharge, and seasonal variation. The monthly 

loads were summed to estimate annual instream 

nutrient load. Organic-nitrogen loads were 

estimated as TKN loads minus ammonia loads, 

and total nitrogen loads were estimated as TKN 

loads plus nitrate loads.

Nutrient Inputs

 Mean total inputs (1986-90) of nitrogen 

in the seven basins ranged from about 11 million kilograms per year (kg/yr) in the Withlacoochee River Basin to 198 million kg/yr in 

the Altamaha River Basin (table 4). Because of 

the differences in the sizes of the basins, comparisons among these seven basins are best discussed in terms of kilograms of nutrient input 

per unit area. The mean nitrogen input per unit 

area for 1986-90 ranged from 2,400 kilograms 

per year per square kilometer (kg/yr)/km2

 in the 

Withlacoochee River Basin to a little over 5,400 

(kg/yr)/km2

 in both the Altamaha and Satilla 

River Basins (table 4). The Altamaha, Satilla, 

and Ochlockonee River Basins had the largest 

amounts of nitrogen introduced per unit area 

(fig. 3, table 4).

Nitrogen from fertilizer (urban and 

agricultural settings) or animal-waste sources 

predominated in all basins. Animal waste constituted the largest source of nitrogen in the 

Altamaha, Satilla, and Suwannee River Basins, 

and fertilizer was the largest source of nitrogen 

in the other four basins (fig. 3). Animal waste 

varied widely in relative importance as a source 

of nitrogen among the seven basins, reflecting 

differences in the numbers of farm animals 

among the basins. As a result of the number of 

broiler chicken cultivation operations, (Florida 

Agricultural Statistics Service, 1992; and Georgia Agricultural Statistics Service, 1993) the 

Altamaha, Satilla, and Suwannee River Basins 

had the highest levels of nitrogen due to animal 

waste. Fertilizer inputs of nitrogen per unit area 

were relatively similar in six of the seven basins 

(fig. 3). Fertilizer inputs of nitrogen per unit area 

were much larger in the Ochlockonee River 

Basin than in the other six basins, probably due 

to the amount of land devoted to agriculture in 

this basin (table 1) and the predominance of 

crops (such as cotton and corn) where large 

amounts of fertilizer are applied (W. Segars, 

Cooperative Extension Service, 1994, oral 

commun.). Interestingly, the Withlacoochee 

Table 3. Nutrient contributions from animal 

wastes

[From R.B. Alexander, U.S. Geological Survey, 

written commun., 1992]

1

Pounds per day per 1,000 pounds of animal.

Animal

Nutrient

component1

Nitrogen Phosphorus

Milk cows 0.400 0.060

Broiler chickens 1.100 0.340

Egglaying chickens 0.830 0.310

Beef cows 0.310 0.105

Hogs 0.280 0.150

Authority, 1993, written commun.). Annual 

fertilizer input for each basin was estimated as 

the sum of inputs for counties included completely or partly in the basin. Where counties 

were partially included in a basin, fertilizer use 

was proportionately distributed based on area 

within the basin. 

Estimates of nitrogen and phosphorus 

inputs from animal manure were calculated for 

1986-90 from the numbers of hogs, beef cattle, 

dairy cattle, broiler chickens, and egg-laying 

chickens in each county (Bureau of the Census, 

1989a,b; Florida Agricultural Statistics Service, 1992; Georgia Agricultural Statistics Service, 1993), and the estimates of the nutrient 

content of manure for each of these groups 

(table 3). Because data were not available for 

1986, data for 1987 were used as an estimate for 

1986. Manure inputs were estimated for the 

study basins by summing the manure loading 

for all the counties contained in a given basin. 

Where counties were partially included in a 

basin, the manure load was proportionately distributed based on county area with the basin. 

These estimates do not consider losses of nitrogen by volatilization that might occur during 

storage, handling, and application. Such losses 

under certain conditions can decrease the nitrogen content of manure by 25 to 80 percent (Kay 

and Hammond, 1985). These estimates do not 

consider nutrient recycling that occurs when 

animals are fed grains grown within the basin.

Estimates of nitrogen and phosphorus 

inputs from septic-tank effluent were calculated 

by multiplying the number of septic tanks in 

each county for 1990 (Bureau of the Census, 

1993a,b) by the estimated average concentration 

of total nitrogen (45 mg/L) and total phosphorus 

(13 mg/L) in septic-tank leachate and then by an 

average effluent volume of 510 liters per day 

(Tchobanoglous, 1991). Where counties overlapped basin boundaries, the number of septic 

tanks was assigned to basins according to the 

estimated percentage of total county population 

residing within the basins.

Estimates of nitrogen and phosphorus 

inputs from wastewater discharges within each 

basin were calculated from water-use data for 

1990 in Florida (Marella, 1994) and Georgia 

(J.L. Fanning, U.S. Geological Survey, 1993, 

written commun.). Wastewater discharge values 

Table 4. Mean nitrogen and phosphorus inputs and stream loads per year and per unit area for the years 1986-90

[Percent stream load to input is circulated using per unit area numbers]

Basin

Nitrogen input Nitrogen

stream load Percent 

stream load 

to input

Phosphorus input Phosphorus

stream load Percent 

stream load 

to input kg×103

/yr (kg/yr)/km2 kg×103/yr (kg/yr)/km2 kg×103/yr (kg/yr)/km2 kg×103/yr (kg/yr)/km2

Altamaha River 198,000 5,470 8,280 228 4.2 50,000 1,380 704 19 1.4

Suwannee River 80,100 3,930 9,700 476 12.1 19,300 946 1,340 66 7.0

St. Johns River 23,600 2,710 3,520 405 14.9 3,670 422 224 26 6.2

Satilla River 39,200 5,430 1,840 255 4.7 9,970 1,380 92 13 1.0

Ogeechee River 18,500 2,690 1,380 200 7.4 3,250 472 107 16 3.4

Withlacoochee River 11,400 2,400 720 152 6.3 1,560 331 39 8 2.4

Ochlockonee River 14,500 4,920 1,080 366 7.5 3,350 1,140 129 44 3.9
5

River Basin, although having a significant 

amount of agricultural land use, had the second 

lowest input of nitrogen from fertilizer.

 Atmospheric deposition contributed less 

than one-fourth of the mean nitrogen input to all 

basins and was consistently the third largest of 

the five sources of nitrogen in all basins except 

the Ogeechee, where it was the second largest 

source (fig. 3). Wastewater-treatment plant 

discharges and septic-tank discharges made the 

smallest contributions (less than 7 percent each) 

to the nitrogen inputs per unit area in all basins 

(fig. 3).

Mean inputs (1986-90) of phosphorus 

(table 4) varied similarly to nitrogen inputs 

between basins (table 4). The mean phosphorus 

input ranged from 331 (kg/yr)/km2

 in the 

Withlacoochee River Basin to a little more than 

1,300 (kg/yr)/km2

 in the Altamaha and Satilla 

River Basins (fig. 4, table 4).

Sources of phosphorus from fertilizer 

(in urban and agricultural settings) and animal 

wastes predominated in all basins (fig. 4). 

Similar to nitrogen inputs, phosphorus inputs 

from animal waste varied more than tenfold 

among the basins. Phosphorus inputs from 

animal waste made up the largest input per unit 

area component in the Altamaha, Satilla, and 

the Suwannee River Basins, whereas phosphorus input from fertilizer was dominant in the 

Ochlockonee, Ogeechee, and St. Johns Rivers. 

Phosphorus inputs from fertilizer were quite 

similar among basins, with the exception of the 

Ochlockonee River Basin which had noticeably 

higher inputs per unit area from fertilizer.

Wastewater-treatment plant discharges 

made small contributions to the mean total 

input of phosphorus when compared to fertilizer and animal wastes in the seven basins 

(fig. 4). Inputs from septic tanks were also 

relatively small except in the St. Johns and the 

Withlacoochee River Basins where they constituted 13 and 16 percent of phosphorus inputs, 

respectively (fig. 4).

Instream Nutrient Loads

 The Suwannee River Basin had the 

largest instream annual load per unit area for 

both nitrogen and phosphorus (fig. 5A). 

Organic nitrogen was the most abundant nitrogen compound, and the highest organic nitrogen loads per unit area occurred in the St. Johns 

River. The highest instream nitrate loads per 

unit area occurred in the Suwannee River followed by the Ochlockonee and the Altamaha 

Rivers (fig. 5B). Instream ammonia loads in all 

seven rivers are relatively low when compared 

to organic nitrogen and nitrate loads (fig. 5B).

The magnitude of annual nutrient loads in 

the seven river basins for the various nutrient 

species appears to be proportional to annual 

river discharge (fig. 6) with several exceptions. 

Nitrate loads are highest per unit area in the 

Suwannee River where ground-water/surfacewater interactions may affect this relation. 

The Altamaha River has high discharge and relatively low nutrient loads, and the Ochlockonee 

River has low discharge and relatively high 

nutrient loads (fig. 6).

The ratio of the mean instream nutrient 

load to the mean nutrient inputs in each basin, 

although not entirely definitive, provides a way 

to compare the nutrient assimilative capacity of 

the seven basins. Comparison of these ratios, 

expressed as percentages for the seven river 

basins, reveals that the instream nitrogen and 

phosphorus loads remaining in these river 

basins are much smaller than the sum of the 

source inputs of these nutrients to these basins 

(table 4). The percentage of nitrogen remaining 

in these rivers ranged from 4.2 to 14.9 percent, 

with significantly higher percentages of 

nitrogen remaining in the St. Johns (14.9 percent) 

and Suwannee (12.1 percent) Rivers than the 

other basins (table 3). Percentages of phosphorus remaining were lower and ranged from 1.0 

to 7.0 percent, with higher percentages of phosphorus remaining in the St. Johns (6.2 percent) 

and Suwannee (7.0 percent) Rivers than in the 

other basins (table 3).

The small fraction of nutrient inputs that 

remain in river water in each of these basins 

shows that each of these river basins is capable 

of assimilating, retaining, and recycling a significant part of the nutrient inputs. Some basins 

are more effective than others in assimilating 

nutrients, and further study is needed to determine the environmental factors that control 

nutrient cycling in each basin.

Figure 3. Mean total nitrogen inputs by 

source for each study basin. 

(Mean computed for years 1986-90.) MEAN PER UNIT AREA, IN KILOGRAMS PER YEAR PER SQUARE KILOMETER

NITROGEN FROM

ATMOSPHERIC DEPOSITION

800

0

200

400

600

NITROGEN FROM

FERTILIZER

4,000

0

1,000

2,000

3,000

NITROGEN FROM

ANIMAL WASTE

0

4,000

1,000

2,000

3,000

NITROGEN FROM WASTEWATERTREATMENT PLANT DISCHARGE

150

0

50

100

ALTAMAHA

SUWANNEE

ST. JOHNS

SATILLA

OGEECHEE

WITHLACOOCHEE

OCHLOCKONEE

BASIN

0

200

50

100

150

NITROGEN FROM

SEPTIC TANKS

Figure 4. Mean total phosphorus inputs by 

source for each study basin.

(Mean computed for years 1986-90.) MEAN PER UNIT AREA, IN KILOGRAMS PER YEAR PER SQUARE KILOMETER

ALTAMAHA

SUWANNEE

ST. JOHNS

SATILLA

OGEECHEE

WITHLACOOCHEE

OCHLOCKONEE

BASIN

PHOSPHORUS FROM

ANIMAL WASTE

0

1,500

500

1,000

PHOSPHORUS FROM

SEPTIC TANKS

0

60

20

40

PHOSPHORUS FROM

FERTILIZER

800

0

200

400

600

PHOSPHORUS FROM

WASTEWATERTREATMENT PLANT

DISCHARGE

0

15

5

10
6

Summary and Discussion

This report compares nutrient inputs in 

river basins with nutrient loads in rivers. There 

was no intent to compute nutrient storage in 

these basins because of the uncertainty and 

magnitude of such a task. Some of the factors 

contributing to the uncertainty of assessing 

nutrient cycling on a basin scale are related to 

processes associated with storage or attenuation 

of nutrients within a river basin and their relative degree of importance. These processes 

include the degree of net biotic production, biological nitrogen fixation, soil sorption, denitrification, and volatilization.

Nutrient inputs and instream loads were 

analyzed and compared for the Altamaha, 

Suwannee, St. Johns, Satilla, Ogeechee, Withlacoochee, and Ochlockonee River Basins in the 

Georgia-Florida Coastal Plain study unit of the 

National Water-Quality Assessment Program. 

The nutrient constituents that were considered 

included nitrate, ammonia, organic nitrogen, 

and total phosphorus; the inputs of nitrogen and 

phosphorus that were considered included 

atmospheric deposition, fertilizer, animal waste, 

wastewater-treatment plant discharge, and 

septic-tank discharge.

 The mean nitrogen input for 1986-90 

ranged from 2,400 (kg/yr)/km2

 in the Withlacoochee River Basin to 5,470 (kg/yr)/km2

 in the 

Altamaha River Basin. The Satilla and Ochlockonee River Basins had nitrogen input totals of 

5,430 and 4,920 (kg/yr)/ km2

, respectively. 

Nitrogen input was predominantly from 

fertilizer (in agricultural and urban settings) or 

animal waste in all basins. Animal waste constituted the largest mean input of nitrogen in the 

Altamaha, Satilla, and Suwannee River Basins, 

and fertilizer was the largest mean input of 

nitrogen in the other four basins. Atmospheric 

deposition contributed less than one-fourth of 

the mean nitrogen input to all basins. Wastewater-treatment plant discharges and septic-tank 

discharges made relatively small contributions 

to the nitrogen inputs per unit area (less than 7 

percent each) in all basins.

The mean total phosphorus input ranged 

from 331 (kg/yr)/km2

 in the Withlacoochee 

River Basin to 1,380 (kg/yr)/km2

 in both the 

Altamaha and Satilla River Basins. The 

Ochlockonee River Basin had a phosphorus 

input of 1,140 (kg/yr)/km2

. 

As with nitrogen, fertilizer or animalwaste sources of phosphorus predominated in 

all basins, and wastewater discharges made only 

small contributions relative to other sources. 

Phosphorus inputs from septic-tank discharges 

however, were somewhat elevated in the 

St. Johns and Withlacoochee River Basins 

(13 and 16 percent of total phosphorus, 

respectively).

The Suwannee River Basin had the highest instream nitrogen and phosphorus loads of 

any of the seven basins, both on a total basis and 

per unit area. Also, the Suwannee River Basin 

had much higher instream nitrate loads per unit 

area than the other six rivers. Instream phosphorus loads were highest in the Suwannee and 

Ochlockonee Rivers. Organic nitrogen loads per 

unit area dominated in all river basins.

Comparison of the ratio of nutrient 

instream loads to inputs, expressed as percentages for the seven river basins, reveals that the 

nitrogen and phosphorus loads leaving these 

river basins in surface-water discharge is a 

small fraction of the nutrient inputs to these 

basins. The highest percentage of nitrogen 

remaining in a basin was that of the St. Johns 

River Basin (14.9 percent) and the lowest was 

the Altamaha River Basin (4.2 percent). The 

highest percentage of phosphorus remaining in 

a basin was that of the Suwannee River Basin 

(7.0 percent), and the lowest was the Satilla 

River Basin (1.0 percent).

The uncertainty of quantifying the 

processes involved with nutrient cycling in each 

of these basins makes predictions of optimum 

natural basin nutrient attenuation extremely 

difficult. Also, there are no definitive means of 

identifying the specific source of nutrients 

observed in river discharge. Nonetheless, the 

examination of nutrient loads in these seven 

river basins in relation to nutrient inputs is 

extremely informative and useful in understanding the relative magnitudes of point and nonpoint sources. Even the lack of some pointsource data that would tend to underestimate 

point source input would not alter the interpretation that nonpoint sources are the major contributor in potentially supplying nutrient inputs 

to river basins in the Georgia-Florida Coastal 

Plain study unit. This report, however, quantifies only the amount of nutrient present in river 

discharge and does not address which type of 

source imposes the most influence to the receiving surface-water body—the direct delivery 

of more concentrated chemicals to the river 

(point sources) or the indirect, more spatially 

dispersed application of less concentrated 

chemicals (nonpoint sources).

Figure 5. (A) Mean instream loads of nitrogen and total phosphorus by basin, 

and (B) mean instream loads of nitrogen species and total phosphorus.

(Mean computed for years 1986-90.) MEAN PER UNIT AREA, IN KILOGRAMS PER YEAR PER SQUARE KILOMETER

500

0

100

200

300

400

INSTREAM LOAD

PER UNIT AREA

NITROGEN

PHOSPHORUS

A

BASIN

ALTAMAHA

SUWANNEE

ST. JOHNS

SATILLA

OGEECHEE

WITHLACOOCHEE

OCHLOCKONEE

400

0

100

200

300

AMMONIA

NITRATE

ORGANIC NITROGEN

B
7

References

Anderson, J.R., Hardy, E.E., Roach, J.T., and Witmer, 

R.E., 1976, A land use and land cover classification system for use with remote sensor data: U.S. 

Geological Survey Professional Paper 964, 28 p.

Baker, L.A., 1991, Appendix B - Regional Estimates of 

Atmospheric Dry Deposition, in Charles, D.F., 

and Christie, S., eds., Acidic deposition and 

aquatic ecosystems: Regional case studies: 

New York, Springer-Verlag, p. 645-652.

Bureau of the Census, 1989a, Census of agriculture, 

1987, Florida state and county data: Washington, D.C., U.S. Department of Commerce, 

Economics and Statistics Administration, 

AC87-A-9 (pt. 9, v. 1), 435 p.

____1989b, Census of agriculture, 1987, Georgia state 

and county data: Washington, D.C., U.S. Department of Commerce, Economics and Statistics 

Administration, AC87-A-10 (pt. 10, v. 1), 698 p.

____1993a, 1990 Census of housing, detailed housing 

characteristics; Florida: Washington, D.C., U.S. 

Department of Commerce, Economics and Statistics Administration, chap. 2-11, 793 p.

____1993b, 1990 Census of housing, detailed housing 

characteristics; Georgia: Washington, D.C., U.S. 

Department of Commerce, Economics and Statistics Administration, chap. 2-12, 506 p.

Cohn, T., DeLong L.L, Gilroy, E. J., Hirsch, R.M., and 

Wells, D.K., 1989, Estimating constituent loads: 

Water Resources Research, v. 25, no. 5, p. 937-

942.

Florida Agricultural Statistics Service, 1992, Livestock, 

dairy, and poultry summary 1991: Orlando, 

Florida Department of Agriculture and Consumer 

Services, 84 p.

Georgia Agricultural Statistics Service, 1993, Georgia 

agricultural facts: Athens, Georgia Department of 

Agriculture, 98 p.

Gilman, C. S., 1964, Rainfall, Section 9, in Ven Te 

Chow, ed., Handbook of Applied Hydrology: 

McGraw-Hill, p. 28-29.

Gilroy, E.J., Hirsch, J.R., and Cohn, T., 1990, Mean 

square error of regression-based constituent transport estimates: Water Resources Research, v. 26, 

no. 9, p. 2069-2077.

Jaworski, N.A., Groffman, P.M., Keller, A.A., and 

Prager, J.C., 1992, A watershed nitrogen and 

phosphorus balance: The upper Potomac River 

Basin: Estuaries, v. 15, no. 1, p. 83-95.

Kay, Furman, and Hammond, Cecil, 1985, Land application of livestock and poultry manure: Athens, 

Cooperative Extension Service, The University of 

Georgia College of Agriculture, Leaflet 378, 4 p.

Marella, R.L., 1994, Estimated discharge of treated 

wastewater in Florida, 1990: U.S. Geological 

Survey Open-File Report 93-364, 53 p.

Mitchell, W.B., Guptill, S.C., Anderson, K.E., Fegeas, 

R.G., and Hallam, C.A., 1977, GIRAS--A geographic information retrieval and analysis system 

for handling land use and land cover data: U.S. 

Geological Survey Professional Paper 1059, 16 p.

Mueller, D.K., Hamilton, P.A., Helsel, D.R., Hitt, K.J., 

and Ruddy, B.C., 1995, Nutrients in ground water 

and surface water of the United States--An analysis of data through 1992: U.S. Geological Survey 

Water Resources Investigations Report 95-4031, 

74 p.

National Atmospheric Deposition Program (NRSP-3)/ 

National Trends Network), 1993, NASP/NTN 

Coordination Office, Natural Resource Ecology 

laboratory: Ft. Collins, Colorado State University.

National Oceanic and Atmospheric Administration, 

1993, The National Coastal Pollutant Discharge 

Inventory--Point Source Methods Document, 

244 p.

Tchobanoglous, George, 1991, Wastewater engineering, 

treatments, disposal, and reuse (3d ed.): New 

York, Metcalf and Eddy, Inc., 2000 p.

U.S. Geological Survey, 1995, Water resources data for 

Florida, water year 1994, v. 1A, northeast Florida 

surface water: U.S. Geological Survey Water-Data 

Report FL-94-1A, p. 60.

Figure 6. Mean annual discharge and annual stream loads by nutrient species 

for each study basin. ANNUAL INSTREAM LOAD, IN KILOGRAMS PER YEAR PER SQUARE KILOMETER

0

600

200

400

ORGANIC NITROGEN

0

300

100

200

NITRATE

1986

1987

1988

1989

1990

1986

1987

1988

1989

1990

1986

1987

1988

1989

1990

1986

1987

1988

1989

1990

1986

1987

1988

1989

1990

1986

1987

1988

1989

1990

1986

1987

1988

1989

1990

ALTAMAHA

SUWANNEE

ST. JOHNS

SATILLA

OGEECHEE

WITHLACOOCHEE

OCHLOCKONEE

BASIN

0

100

20

40

60

80

TOTAL PHOSPHORUS

0

40

10

20

30

AMMONIA

DISCHARGE, IN CUBIC

METERS X 109 PER YEAR

15

0

5

10

DISCHARGE
8

For more information,

please contact:

Project Chief

Georgia-Florida Coastal 

Plain Study Unit

U.S. Geological Survey

Suite 3015

227 North Bronough St.

Tallahassee, FL 32301

(850) 942-9500

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey

Copies of this report 

can be purchased 

from:

U.S. Geological Survey

Branch of Information 

Services

Box 25286

Denver, CO 80225

(800) USA-MAPS

The NAWQA Program

The National Water-Quality Assessment (NAWQA) Program, conducted by 

the U.S. Geological Survey, evaluates the Nation’s ground water, rivers and 

streams. The building blocks of the Program are scientific investigations in 59 

hydrologic basins (“study units”). Collectively, these study units cover one-half 

of the land area of the United States, encompass more than two-thirds of the water 

resources used in human activities, and include sources of drinking water used by 

70 percent of the U.S. population served by public supplies.

The Program’s investigative methods balance the unique characteristics of 

individual watersheds with a nationally consistent emphasis on physical, chemical 

and biological evidence. Study-unit investigations proceed in 3 to 5 year rotations, 

with about one-third of the study units involved in intensive sampling at any given 

time. Each basin is revisited every 10 years to record trends.